The present invention is directed to a particular type of nozzle for the injection of a molten plastic substance into one or several molding cavities.
The present invention is applicable preferably to nozzles used in machines adapted for molding, at the same time, a plurality of plastic articles, i.e. the so-called xe2x80x9cpreformsxe2x80x9d, that are intended for subsequent processing by blow molding into appropriate final containers, especially plastic bottles.
Although reference will be made in the following description, mainly for reasons of greater descriptive convenience, to a vertical-drive machine for molding preforms of plastic material, as this is used in combination with the preliminary steps of melting and extrusion of the plastic material, it will be appreciated that the present invention shall be understood as applying also to horizontal-drive molding machines or even to other uses, as far as these fall within the scope of the appended claims.
It is generally known in the art that, during a preform molding operation, use is generally made of a molding machine comprising a nozzle-carrying plate and a plate provided therebelow for the so-called hot chambers (or hot runners, as they are more particularly known in the art). This plate generally contains the hot runners, i.e. the conduits provided to transfer the molten resin from an appropriate extruder. The cavity-carrying plate contains a plurality of cavities into which the flow of plastic resin flowing in from the base plate is injected. The flow of plastic resin is transported between the elements by appropriate respective injection nozzles arranged between the cavity-carrying plate and the nozzle-carrying plate.
The means that are commonly used to hold the nozzles in close contact with the surfaces of the plates include providing the nozzles with threads in view of then fastening them by screwing in, or, as an alternative thereto, generating a force between the parallel surfaces of the nozzles and the surfaces of the plates, in such a manner as to enable the nozzles to be locked in position therebetween by pressure.
All such measures are well-known to those skilled in the art and are briefly discussed herein for the sole purpose of more effectively introducing the technical context which the present invention is referring to.
One of the most significant problems concerning the design and the utilization of such nozzles arises from the circumstance that these nozzles are assembled under cold conditions and must start operating with the other parts of the machine while not yet at their rated working or steady-state temperature, without giving rise to losses of molten resin, not even in the initial phases of the process, when the installation has not yet reached its steady-state working conditions.
During operation, the plates that are coupled with the nozzles heat up and, as a result, tend to expand, thereby reducing the available space for the nozzle and, therefore, a compression force may develop and damage the mutually involved parts.
In view of reducing such a compression effect, the need arises for an initial coupling that is not overly tight. This, however, might well lead to initial losses of resin, i.e. a situation that should on the contrary be prevented in any case, due to the need for such leaking resin to be immediately removed, in order to avoid serious losses caused by machine downtime and the necessary personnel for performing such a task.
Nozzles for the injection of plastic substances of the kind illustrated in FIGS. 1 and 2 are known to have been disclosed and to be normally used in the art. Such nozzles include a front central portion 101 and a rear annular portion 102. A groove 103 is provided in the rear annular portion 102 so as to delimit an external wall 104.
When the nozzle is normally coupled between the nozzle-carrying plate 30 and the plate 40 containing the hot runners, the nozzle is subjected to mutually opposing forces indicated by the letters xe2x80x9cFxe2x80x9d and xe2x80x9cGxe2x80x9d in the Figures.
Due to the structure of the nozzle, the latter withstands the mutually contrasting action of these forces through the compression of the wall 104 and the rear solid portion of rear annular portion 102.
Since the latter is solid, it inherently has a very high compressive strength and, therefore, a practically nonexistent ability to deform axially. As far as the wall 104 is concerned, it has a rather small axial dimension and, as a consequence, a correspondingly small compressibility, even if subjected to an elevated compression. In practice, therefore, such nozzles eventually offer a reduced ability to deform elastically and, as a result, a reduced ability to take up the dimensional deviations that are introduced between the plates by the thermal expansion processes thereof.
The state of the art concerning nozzles that are pressure fitted and locked between plates is disclosed in the following patent publications: U.S. Pat. No. 4,588,367; U.S. Pat. No. 5,220,154; U.S. Pat. No. 5,554,395; U.S. Pat. No. 5,759,595; and U.S. Pat. No. 5,820,899.
All of these patent publications address, according to a variety of approaches, the issue of finding the best possible solution to the problem of coping with a number of mutually clashing requirements, i.e.:
ensuring protection when starting from a cold condition
the wide range of dimensional tolerances of both plates and nozzles; and
the capability of operating within a wider temperature range than the range normally involved or used, including the possibility of reaching temperatures of up to 50xc2x0 C. above the normal operating temperature.
However, none of the cited patents appears to be able to solve all of the above mentioned requirements to a satisfactory extent. Furthermore, some of them even call for the use of supplementary sealing means, such as gaskets and the like, which do not solve the problem in any permanent manner, but rather tend to introduce further complications.
Based on the above considerations, it is therefore a main purpose of the present invention to provide a nozzle that enables all of the above three mentioned mutually clashing requirements to be complied with automatically.
A further purpose of the present invention is to provide a nozzle of the above cited type, which is capable of being easily implemented, used and operated through the use of readily available and, therefore, cost-effective materials and techniques.
Such objects of the present invention, along with further features thereof, are achieved with a nozzle that is constructed and operates as recited in the appended claims.
The present invention may take the form of a preferred, although not sole embodiment such as the one that is described in detail below by way of a non-limiting example with reference to the accompanying drawings, in which: